A computer network is a collection of interconnected computing devices that exchange data and share resources. In a packet-based network the computing devices communicate data by dividing the data into small blocks called packets. Certain devices within the network, such as routers, maintain routing information that describes routes through the network. In this way, the packets may be individually routed across the network from a source device to a destination device. The destination device extracts the data from the packets and assembles the data into its original form. Dividing the data into packets enables the source device to resend only those individual packets that may be lost during transmission.
A virtual private local area network service (VPLS) is one example of a layer two (L2) virtual private network (VPN) service that may be used to extend two or more remote customer networks, i.e., VPLS sites, through an intermediate network (usually referred to as a provider network), such as the Internet, in a transparent manner, i.e., as if the intermediate network does not exist and the remote customer networks are instead directly connected. In particular, the VPLS transports L2 communications, such as Ethernet packets, between customer networks via the intermediate network. In a typical configuration, provider edge (PE) routers coupled the customer networks operate as ingress and egress for label switched paths (LSPs) or other tunnels that may be used as pseudowires within the provider network to carry encapsulated L2 communications as if these customer networks were directly attached to the same local area network (LAN). Such PE routers will be referred to as “members of the VPLS domain” in that they maintain L2 state information for the VPLS service. In Border Gateway Protocol (BGP)-based VPLS networks, the PE routers use BGP as the control plane protocol for signaling the VPLS service.
In some cases, a VPLS site may be given redundant connectivity to a network through multiple PE routers, which is referred to as “multi-homing.” In multi-homing, one of the multiple PE routers coupled to the VPLS site is chosen as the “designated forwarder” (DF) to send traffic to and from the VPLS site. The other one or more PE routers are designated as backup forwarders which can be used to send traffic to and from the VPLS site in the event of a network failure that would preclude the current DF from sending the traffic.